Project Summary Sarcomere in striated muscle is the basic unit of contractile apparatuses. Assembly and maintenance of organized sarcomeric structures are essential for proper contraction and relaxation in muscle. Actin is one of the major components of sarcomeric thin filaments, and length and orientation of the filaments are precisely regulated in striated muscle. However, the mechanism of assembly and maintenance of sarcomeric actin filaments is complex and poorly understood. A number of regulators of actin dynamics have been identified in skeletal muscle, and some of them are linked to genetic muscle disorders. Nemaline myopathy involves formation of abnormal actin-rich aggregates or rods in skeletal muscle and is caused by mutations in actin or regulators of actin dynamics. Therefore, the regulation of actin dynamics is fundamentally important for building functional contractile apparatuses in skeletal muscle, and malfunction in this system leads to muscle disorders. To investigate the regulatory mechanism of actin dynamics in striated muscle, we use the nematode Caenorhabditis elegans as a model system. Body wall muscle of C. elegans is striated muscle, and most of sarcomeric proteins are conserved between C. elegans and humans. In C. elegans, we have identified that ADF/cofilin (UNC-60B), AIP1 (UNC-78 and AIPL-1), and cyclase-associated protein (CAS-1) enhance turnover of actin filaments and essential for organized assembly of sarcomeric actin filaments. In addition, we identified SUP-13 as a new regulator of muscle actin. Furthermore, we obtained evidence that the barbed ends of sarcomeric actin filaments are aligned to previously unrecognized Z-line-like structures in C. elegans muscle. The central hypothesis of this project is that proper site-specific regulation of actin filament dynamics is required for assembly and maintenance of sarcomeric actin filaments. We propose three aims: (Aim 1) to determine how sarcomeric actin assembly is regulated by ADF/cofilin, AIP1, and SUP-13, (Aim 2) to determine how actin filament dynamics are regulated by cyclase-associated protein, and (Aim 3) to determine how sarcomeric actin filaments are anchored near their barbed ends. We expect that results of this research will provide new insight into the regulation of actin dynamics in striated muscle. Most of the actin-regulatory proteins studied in this project are also expressed in mammalian skeletal muscle, and some of them are involved in genetic muscle disorders in humans. We hope that our research in a model organism will help to understand the conserved mechanism of myofibril assembly and maintenance under normal and pathological conditions.